Electrical motors for industrial use often have housings or frames that totally enclose the motor winding and armature areas. While a housing protects an electric motor when exposed to environments contaminated with moisture or dust, for example, a cooling fan and specially designed housing surfaces are necessary to carry away heat transmitted to the housing by the electrodynamically operating components of the motor. In one construction of such a fan-cooled motor, the motor housing is generally cylindrical and the fan is mounted at the end of the housing opposite the main drive shaft from the motor. For safety and to ensure proper air flow, a cup-like cap or "bell" is fitted over the fan and the adjacent end of the motor housing. The fan draws air in through a grille in the center of the cap and exhausts the air along the length of the motor housing between the housing and the sides of the cap. A plurality of cooling fins project from and extend lengthwise of the motor housing to improve the cooling of the motor.
As the fan blades rotate to produce the required air flow, the noise emanating from the motor increases beyond the level normally generated by the motor itself. The additional noise is caused by air flowing through various passages about the motor housing and by the movement of the fan blades. To maintain the noise level within acceptable limits, the fan inlets for air-cooled electric motors can be provided with silencers. A typical silencer is a cylindrical member incorporating sound absorbing material and mounted to extend axially away from a fan inlet. Air is drawn into the silencer either axially or radially and then flows axially through the silencer to the fan. Such silencers are manufactured commercially by The Spencer Turbine Company of Hartford, Connecticut, for example. Other silencing devices incorporating sound absorbing material are described and illustrated in Guthrie U.S. Pat. No. 2,056,517 and Curzon U.S. Pat. No. 2,505,854.
Another type of silencer for a fan-cooled electric motor is described and illustrated in Wall U.S. Pat. No. 2,881,337. In the Wall device, which does not utilize sound absorbing material, the end cap or "bell" of an electric motor is replaced with a combination cap or casing and muffler. The combination member has a cylindrical shell and is internally partitioned into a pair of resonant chambers. The chambers both have central openings axially aligned with the motor frame or housing. The cooling fan for the motor draws in air axially through the resonant chambers and discharges the air along the exterior surface of the motor housing. In one embodiment, the shell of the combination member extends the full length of the motor and terminates in a resonant-type muffler to attenuate the noise from the discharge of the cooling fan. As is suggested by the patent, such a resonant-type muffler is effective for only a limited and predetermined range of frequencies.